Was it letter 6475? That tells you how much they sent to you. Double check your bank statements and check your IRS account to see when and what they sent you.
https://www.irs.gov/payments/your-online-account
If you didn't get the 3rd Stimulus check but the IRS shows they sent it to you then request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
